Copyedit generics.md and traits.md

Squashed at reviewer's request:

Add heading at the end of the introductory material
Spice up introductory paragraphs a bit
Use quotes instead of <code> for phrase
Remove "other" in "other restrictions" (it's not obvious that any other
restrictions have been mentioned)
"Default methods" is a second-level heading, but is not a subsection of
"Where clause"
Reword "Default methods" introduction: it's not the "last feature" on
this page

Author: lastorset

src/doc/trpl/generics.md

@@ -6,7 +6,7 @@ Generics are called ‘parametric polymorphism’ in type theory,
 which means that they are types or functions that have multiple forms (‘poly’
 is multiple, ‘morph’ is form) over a given parameter (‘parametric’).
 
-Anyway, enough with type theory, let’s check out some generic code. Rust’s
+Anyway, enough type theory, let’s check out some generic code. Rust’s
 standard library provides a type, `Option<T>`, that’s generic:
 
 ```rust
@@ -27,7 +27,7 @@ let x: Option<i32> = Some(5);
 
 In the type declaration, we say `Option<i32>`. Note how similar this looks to
 `Option<T>`. So, in this particular `Option`, `T` has the value of `i32`. On
-the right-hand side of the binding, we do make a `Some(T)`, where `T` is `5`.
+the right-hand side of the binding, we make a `Some(T)`, where `T` is `5`.
 Since that’s an `i32`, the two sides match, and Rust is happy. If they didn’t
 match, we’d get an error:
 

src/doc/trpl/traits.md

@@ -47,8 +47,11 @@ As you can see, the `trait` block looks very similar to the `impl` block,
 but we don’t define a body, just a type signature. When we `impl` a trait,
 we use `impl Trait for Item`, rather than just `impl Item`.
 
-We can use traits to constrain our generics. Consider this function, which
-does not compile:
+## Traits bounds for generic functions
+
+Traits are useful because they allow a type to make certain promises about its
+behavior. Generic functions can exploit this to constrain the types they
+accept. Consider this function, which does not compile:
 
 ```rust,ignore
 fn print_area<T>(shape: T) {
@@ -75,7 +78,7 @@ fn print_area<T: HasArea>(shape: T) {
 }
 ```
 
-The syntax `<T: HasArea>` means `any type that implements the HasArea trait`.
+The syntax `<T: HasArea>` means “any type that implements the `HasArea` trait.”
 Because traits define function type signatures, we can be sure that any type
 which implements `HasArea` will have an `.area()` method.
 
@@ -154,8 +157,8 @@ error: the trait `HasArea` is not implemented for the type `_` [E0277]
 
 ## Traits bounds for generic structs
 
-Trait constraints also can apply to implementations for generic structs.  Just
-append the constraint when you declare type parameters. Here is a new type
+Your generic structs can also benefit from trait constraints. All you need to
+do is append the constraint when you declare type parameters. Here is a new
 type `Rectangle<T>` and its operation `is_square()`:
 
 ```rust
@@ -232,7 +235,7 @@ impl HasArea for i32 {
 It is considered poor style to implement methods on such primitive types, even
 though it is possible.
 
-This may seem like the Wild West, but there are two other restrictions around
+This may seem like the Wild West, but there are two restrictions around
 implementing traits that prevent this from getting out of hand. The first is
 that if the trait isn’t defined in your scope, it doesn’t apply. Here’s an
 example: the standard library provides a [`Write`][write] trait which adds
@@ -397,10 +400,10 @@ This shows off the additional feature of `where` clauses: they allow bounds
 where the left-hand side is an arbitrary type (`i32` in this case), not just a
 plain type parameter (like `T`).
 
-## Default methods
+# Default methods
 
-There’s one last feature of traits we should cover: default methods. It’s
-easiest just to show an example:
+If you already know how a typical implementor will define a method, you can
+let your trait supply a default:
 
 ```rust
 trait Foo {